(1) Field of the Invention
The invention relates to the field of sensors and methods for measuring one or more select components of a material. In particular, the invention relates to measuring the components by emitting electromagnetic radiation at the material and detecting the amount of emerging radiation at separate locations. The invention can accurately measure the components (e.g., moisture) of different grades of paper by eliminating the effects of the scattering power and determining absorption power at each band of the spectrum necessary for the particular measurement.
(2) Description of the Related Art
Because paper is produced in a sheet from an aqueous suspension, which includes wood pulp fibers, cotton fibers and various chemicals, it initially contains a considerable amount of moisture. Most of this moisture is removed during paper production. However, for a variety of reasons, it is often desirable to include at least some moisture in the paper. For example, if the paper is too dry, it will tend to curl at the edges or may increase the cost of production.
A paper sheet is typically dried by passing it around heated drying drums. However, this tends to dry the sheet unevenly across its cross-direction width, producing paper of uneven quality. Devices have been developed to selectively moisten or dry the cross-directional sections of the sheet. Boissevain et al. U.S. Pat. No. 5,020,469, assigned to Measurex Corporation, describes such a device. Typically, the moistening or drying occurs after the sheet has passed around the drying drums. Of course, the paper mill operator, or the paper mill's process control computer, must determine the cross-directional moisture profile of the sheet before these devices can be used effectively. Thus, moisture sensors have been developed to measure the cross-directional moisture profile.
Water absorbs electromagnetic radiation across the infrared spectrum as a function of wavelength. Some moisture sensors take advantage of this phenomenon by emitting infrared radiation at the sheet and detecting the amount of the radiation passing through or reflected from the sheet at or near the water absorption peak. The more moisture in the sheet, the less radiation at or near the water absorption peak that will pass through or be reflected from the sheet.
An infrared moisture sensor can be set up with an infrared radiation source located on one side of the sheet and two detectors on the opposite side. Each detector has an associated band pass filter positioned between the source and the detector so that the detector only receives radiation in a select band of the spectrum. A first band pass filter passes that portion of radiation which is near a water absorption peak to a first detector. Thus, the first detector is primarily sensitive to the amount of water in the sheet and receives more infrared radiation when the sheet is dry and less infrared radiation when the sheet is moist.
A second band pass filter passes radiation in a band of the spectrum where there is less moisture absorption. In this band, most of the absorption is from sheet fibers rather than moisture in the sheet. Thus, when the basis weight (i.e., weight per unit area) of the sheet fiber increases, the second detector receives less infrared radiation. The output of the second detector corrects for changes in the basis weight of the sheet fiber. When the outputs from these two detectors are properly combined, the sensor provides an accurate measurement of the moisture in the sheet so that the changes in the basis weight of the sheet fiber do not affect the moisture measurement.
Howarth et al. U.S. Pat. No. 4,928,013, assigned to Measurex Corporation, describes an infrared moisture sensor of this type with two band pass filters that are selected to compensate for sheet temperature changes which shift the absorption spectrum to either shorter or longer wavelengths. In this sensor, a first band pass filter, associated with a measure detector, is selected so that it is surrounds the water absorption peak at about 1.93 microns. When the sheet temperature increases, the intensity of radiation increases at the long wavelength side of the pass band filter while an approximately equal decrease occurs at the short wavelength side. Accordingly, the amount of infrared radiation reaching the measure detector remains substantially constant when the sheet temperature changes. A second band pass filter, associated with a reference detector, is selected so that it is in a band of the infrared spectrum that is predominantly absorbed by the sheet fibers. The intensity of the radiation detected by the reference detector primarily indicates the basis weight of the sheet.
However, the intensity of the detected radiation is not only dependent upon the moisture, basis weight and temperature of the sheet. Each grade of sheet has its scattering and absorption powers that affect the intensity of the detected radiation. A scattering power of a material defines its ability to change the direction of light incident upon the material from either the line of incidence when transmitted through or from a specular direction when reflected from the material. An absorption power defines the material's ability to absorb the incident light rather than allow it to be transmitted through or reflected from the sheet.
The source of the wood fiber used to make paper products may affect the value of the scattering coefficient and/or the broadband absorption coefficients. This in turn may affect the accuracy of an infrared moisture sensor. Changes in the scattering power of paper are often caused when the source of the paper pulp changes from one species of wood to another or from virgin to recycled fiber. Broadband absorption change may be caused by the carbon black in printer inks used in recycled paper or added to colored paper.
Howarth U.S. Pat. No. 3,793,524, assigned to Measurex Corporation, describes an infrared moisture sensor for measuring the moisture of a sheet of material such as paper. The moisture sensor includes an infrared source that directs infrared radiation out of an aperture through paper and into another aperture to a detector. The source and detector apertures are located in opposing reflective paper guides disposed on either side of the paper and are offset from one another so that the radiation is reflected repeatedly back and forth between the paper guides in traveling from the source aperture and to the detector aperture (FIG. 2). The offset geometry results in relatively low sensitivity to the scattering power of the paper but may require calibration to measure different grades of paper.
Tamura et al. U.S. Pat. No. 4,345,150 ("Tamura") describes a moisture meter. As shown in FIG. 8A, the moisture meter is shown with an irradiation aperture 4 having an optical axis aligned with that of the incident aperture 5 but not (or offset from) with that of incident aperture 5'. As a result, signals R.sub.t and M.sub.t are generated from the light which has been incident upon the aperture 5 and signals R.sub.n and M.sub.n are generated from light which has been incident upon aperture 5'.
Because paper includes both elements which scatter and absorb light, the signals R.sub.5, M.sub.t, R.sub.n and M.sub.n will be sensitive to both the scattering and absorption powers of the paper being measured. Tamura fails to recognize the problem of the scattering power affecting these signals and thereby the measurements or describe any technique for determining the absorption power of the paper by itself.
Tamura's moisture meter would therefore require a number of calibrations to measure all grades of paper, otherwise, the scattering power affects the accuracy of the moisture measurements. It would be highly desirable to have a moisture sensor which has one calibration for a broad range of grades of paper. To achieve this goal any sensitivity to the scattering power must be eliminated.